Alternating current motor control



April 20,1954 M. c. SPENCER 2,676,292

' ALTERNATING CURRENT MOTOR CONTROL Filed Aug. 1, 1944 Sheets-Sheet 1 I l in: Vo/Zayc P55 CENT JYNCHRONOUS JPEED 1 I00 200 PER CEA/T FOIL (040 7'0 TORQUE Pf? C EN 7' JYNCH/PO/VOUJ SPEED I00 PEI? CEA/fFl/(L 2040 TORQUE 100% Line Voltage 30 I00 2 200 I I PEI? CENT FULL LOAD TORQUE INVENTOR Mill/7RD COLE SPENCEf? ATTORNEYS PE? CEN T \SYNCHEONOUS SPEED M. C. SPENCER ALTERNATING CURRENT MOTOR CONTROL April 20, 195 4 5 Sheets-Sheet 2 Filed Aug. 1, 1944 N E MR M n P April 20, 1954 M. c. SPENCER 2,676,292

ALTERNATING CURRENT MOTOR CONTROL Filed Aug. 1, 1944 5 Sheets-Sheet 3 INVENTORQ Mam/w can s a/cm l -W Mann:- ATmRNEY-S April 20, 1954 M. c. SPENCER ALTERNATING CURRENT MOTOR CONTROL INVENTOR.

M/LLA/PD (OZE5PENCER BY I A TTORNE Y5 April 2 1954 M. c. SPENCER 2,676,292

ALTERNATING CURRENT MOTOR CONTROL Filed Aug. 1, 1944 5 Sheets-Sheet 5 INVENTOR. M/LLAAD COLE SPENCER 7 M, w wm ATTORNEYS Patented Apr. 2Q, 1954 2,676,292 ALTERNATING CURRENT MOTOR CONTROL Millard Cole Spencer, East Orange, N. J assignor,

by mesne assignments,

to Elliott Company,

Jeannette, Pa., a corporation of Pennsylvania Application August 1, 1944, Serial No. 547,526 27 Claims. (01. 318-229) The satisfactory control of the speed of alternating current motors is well known to be dlfficult, and, therefore, it has been customary in the art to employ expedients in the nature of compromises, or to avoid the problem altogether by the use of direct current motors. However, the advantages inherent in alternating current power systems, and likewise in alternating current motors are so great that the commercial development of alternating current systems has greatly outgrown that of direct current systems. One of the chief advantages in direct current power systems, which prior to the present invention has not been satisfactorily duplicated in alternating current systems, has been the facility with which the speed of direct current motors could be controlled. By means of this invention it now becomes possible not only satisfactorily to control the speed. of alternating current induction motors, but likewise the direction of-rotation of such motors. By means of the invention it is feasible to control the speed of single phase and multiphase motors, and the direction of rotation and speed of multi-phase motors.

Many of the advantages of an alternating current motor of which the speed as Well as the direction of rotation may readily be controlled, remotely if desired, will at once occur to those skilled in the art. However, a few of the more important advantages introduced by the present invention may here be stated. The motor may, if desired, be of the squirrel cage type which is of great simplicity, eliminating commutator, brushes and slip rings. The speed may be adjusted at will from zero to full speed continuously throughout the speed range with shunt motor characteristics, viz., at each speed setting of the control, the motor speed is substantially independent of the load, and, furthermore, operation at full load torque at all speeds is possible. With but a single control device, the motor may be reversed and the same speed control attained in the reverse direction. High loads (for example 200% or more) can be carried for a substantial period of time because the control system per se does not carry the motor current, and high starting torques (for example 300%) can be developed. For the same reason almost all of the components of a given control system in accordance with the invention may be employed with motors of any power rating. Finally, by means of the present invention the motor may be stopped and started and the speed controlled from zero to maximum, in either direction, and remotely if desired, without opening or closing the power line to the motor and without actuating any moving parts which carry motor current.

The number of uses to which motors having the advantages of control above described may be put is very great, but among them may be mentioned drives for machine tools, knitting machines, fans and centrifugal pumps, gun turrets both for ships and aircraft, and a wide variety of traction applications.

Briefly, the invention comprises an induction motor of which the secondary winding or rotor preferably has certain characteristics in respect to those of the load, a reactor in series in each lead between the line and the motor stator winding, and a novel control circuit therefor whereby the speed of the motor may be adjusted and controlled over a wide range of speed and load by adjusting the voltage applied to the motor stator winding by varying the effective reactance of the reactor or reactors. In a preferred modification the invention comprises means including at least one direct-current controlled saturable core reactor, and electronic components for automatically deriving a direct current voltag proportional to the speed of the motor to be controlled so that if this voltage becomes more or less than that representing the preselected speed the voltage applied to the motor will be automatically varied by the reactor accordingly, to effect the necessary speed control. Automatic regulation within very close limits can thus be attained. Furthermore, since the power circuit includes no electronic tubes, the starting current and starting torque are not limited thereby. If reversal of the direction of rotation be desired together with speed control in the reverse direction it is necessary to duplicate certain of the components of the system and interconnect them through the necessary elements in common. The invention includes many modifications, some of which herein described provide intermediate types of control which are entirely satisfactory for certain applications.

The invention will be more readily understood by a consideration of the following description taken together with the accompanying drawings, wherein:

Fig. 1 is the circuit diaphragm of a simple form of control circuit employing saturable core reactors as applied to an induction motor included here for purposes of explanation;

Fig. 1c shows the nature of one of the saturable core reactors illustrated in l and in many of the other figures;

Figs. 2, 3 and l represent load curves and speed vs. torque curves of induction motors of different characteristics including those uitable for use with the control system of the invention;

Fig. 5 is a circuit diagram, also for purposes of explanation, of an induction motor of whic the speed is automatically maintained at a preselected value by means of 'a centriiugally operated switch which operates a control circuit based on that of Fig. 1;

Fig. 6 is the circuit diagram of a system which operates similarly to that of Fig. 5, but in which the centrifugal switch is replaced by electronic and other components in accordance with the invention, and in which the direct current which controls the operation of the system is derived as a function of the motor speed;

Fig. 6c is an alternative modification or" Fig. 5 in which the tachometer is a direct-current inachine;

The system of Fig. '7 is a refinement of that of Fig. 6 and is capable of closer reg1 aticn and control;

Fig. 8 is a circuit diagram of a preferred embodiment of the invention by which the speed and direction of rotation of an induction root-or will be automatically maintained and may be accurately controlled by manipulation a single control means; and

Fig. 9 is the circuit diagram of simplified system by which the speed and dire tion of rotation of an induction motor may be ually controlled, but without the automatic feature included in the system of Fig. 8.

A simple type of control system not including the invention will first be referred to so that the practical application of the more theoretical discussion which follows will be better understood.

Referring, therefore, to l, the stator windings Etc, 2?) and 2c of an induction "motor shown in the usual Y-connection The rotor i is represented at the center. Between each of these stator windings and the line terminals lilo, i229 and tile a saturable core reactor 5a, lib and to, r .spectively, connected. Oi? course, only as many reactors the e are phases would be user.. The nature of the r s nors herein suggested as a suitable type shown in more detail in 1a and. is device well known in the art. The reactors are similar each in cludes a closed core with three legs. A coil 3 on the center leg of the core is for direct current. A winding i, split into two sections and designed to carry the entire motor current for the phase with which that reactor connected in series, is located on the two outer legs ol the core. The two sections of winding @l are so connected that there is no inductive ct between this winding and the direct current coil on the center leg of the core. The of the winding l depends upon the effect ve magnetic permeability of the core which, in turn, depends upon the magnetic saturation of the core as determined by the amount of direct current which flows through the direct-current coil on the center leg of the core. The result of this con structicn is that wh n no direct-current flows through coil 3 the reactor introduces maximum reactance in the circuit, whereas with sumcient 4 direct current flowing in coil 3 this reactance is reduced to a minimum, so that by varying the value of direct current from zero to a suitable maximum value the reactance and hence the voltage drop across it may be varied over such a wide range that the motor speed may be continuously controlled from zero to full speed. In other words the effective voltage applied to the motor will be balanced regardless of the number of phases, and may be continuously controlled from substantially zero to full line voltage because the line voltage is reduced by voltage drop the reactor. The lower the voltage ap plied to the motor, the lower will be the motor speed, and conversely. Hence the motor s eed is a function of and is approximately proportional to the value of direct current flowing through the direct current coils of the reactors. ihe invention is not limited to the type of. reactor illustrated, because any other type of functionally equivalent reactor could be substituted.

In Fig. i a suitable source of direct current for controlling the reactance oi": the above described reactors is represented to comprise two gas-iilled rectifier tubes 8 and 9 connected to a suitable power transformer ill to provioe lull-wave ctification furnishing eilectively direct current to the terminals 8- and l, respectively, from a midtap on secondary 2i and a common connection to the cathodes of tubes 8 and ii. To vary the direct current, rheostat I! is connected in series in the direct current circuit.

Those skilled in the art will recall that the control of the speed of an induction motor by Variation oi"- the applied voltage has heretofore been considered to be unsatisfactory and therefore not recommended. This finding has been based on the fact that attempts so to control the speed. of induction motors has resulted in extremely poor speed regulation, low starting torque and considerable over-heating of the motor at low speeds. The reason. for this poor perform ance of a standard induction motor with speed control by variation of the applied voltage, and the means whereby the motor characteristics can be improved and made entirely satisfactory, can perhaps best be illustrated by the curves oi Figs. 2, 3 and 4 and the data in Tables and "ill, below In considering these figures and data it is well to keep in mind the fact that the torque of an cluction motor is proportional to the magnetic flux multiplied by the rotor current which is in phase with the flux; that the flux is approximately proportional to the line vol age and that the slip is proportional to the ratio of the copper loss in the rotor to the total power received by the rotor from the stator. Likewise, normal rated torque is the torque which the motor will deliver continuously with the motor operating at rated. voltage and rated speed with a maximum rise in temperature of any part not exceeding a predetermined amount, measured by thermometer, preferably 40 degrees centigrade. By the same token, normal rated current is the cur rent which the motor draws when operated at normal rated torque, normal rated speed, and normal rated voltage.

The curves of Fig. 2 represent speed vs. torque characteristics of a motor of the squirrel cage type having a standard low resistance rotor, at five diiierent fractions (in per-cent) or full line voltage. The dash curve marked (A) represents the torque required by a load which required a constant torque of 190 per-cent at all speeds. It is seen that the we per-cent line voltage curve cuts the load torque line at two points; that is, at two different speeds the torque developed by the motor is exactly equal to the torque required by the load. At the upper point of intersection the motor speed would be stable. If the motor speed increased slightly the torque developed by the motor would be less than the torque required by the load and the motor would be slowed down by the load. If the motor slowed down slightly below the point of intersection the torque developed by the motor would be slightly mor than the torque required by the load and the motor would speed up. The motor would therefore tend to run at exactly the speed corresponding to the point of intersection and the speed would be stable.

At the lower point of intersection the reverse would be true, viz., the motor speed would be unstable. At a slight increase in Speed above the lower point of intersection the torque developed by the motor would be greater than the load torque and the motor speed would increase on up to the speed corresponding to'the upper point of intersection. At a slightly lower speed than that at the lower point of intersection the motor torque becomes less than the load torque and the motor would slow down to standstill. Therefore, it is evident that at the lower point of intersection the motor speed is unstable.

It is seen from Fig. 2 that at the upper or stable speed point the motor speed, with 100 per cent line voltage and a load of 100 per cent torque, is very nearly equal to 100 per cent or synchronous speed. It is also seen that, if the motor voltage is reduced to so per cent, there is only a very small reduction in speed. The point of intersection of the 80 per cent torque curve with the load line, A, is only slightly lower than the intersection with the 100 per cent voltage curve. Even a reduction to to per cent line voltage causes only a small reduction in speed. It is further seen that if the motor voltage is reduced much below 60 per cent the motor torque curve would fail entirely to cut the 100 per cent load curve, and under this load the motor would not continue to run at all but would slow down to standstill. Therefore, a motor of this standard induction type can be slowed down only a few per cent in speed, by a reduction of the motor voltage, before the motor will fail to carry the load entirely and slow down to standstill.

Somewhat better results can be obtained by increasing the rotor resistance so that the speed vs. torque curves will be as shown in Fig. 3. Here, a reduction of the motor voltage to about 50 per cent of the line voltage Will cause a considerable reduction in motor speed but at this voltage the motor torque curve coincides with the load line, A, over the lower speed range, and the motor speed is therefore unstable over this portion of the range.

With a still further increase in rotor resistance motor speed vs. torque curves such as shown in Fig. 4 can be obtained. Here it is seen that the motor torque curves nowhere coincide with a motor load curve and cut the motor load curves at a substantial angle at all speeds from the maximum speed at 190 per cent voltage down to zero speed at about 50 per cent voltage. The speed of such a motor, driving a constant torque load, can, therefore, be adjusted to any value from a maximum speed down to zero speed by adjusting the motor voltage, and stable speed operation obtained at each speed setting.

Not only is it impractical to reduce the speed of a motor which has a low resistance rotor by a reduction in the motor voltage, because after a reduction of a few per cent in speed the motor speed suddenly drops to zero, but to obtain even a small reduction in speed the motor draws so much current that it overheats. This is illustrated by the data in Table I, which, like that of Table II, is given by way of illustration only, this data being a composite of a series of measurements and therefore differing slightly from the data on which the curves of Figs. 2, 3 and 4 were drawn:

Table I Rotor Re- Rotor 1 R Ro- Sh Flux, Percent sistance, Current, tor Loss, fi

hms Amperes Watts It is assumed that the rotor has a resistance of .09044 ohm and that a rotor current of 300 amperes is required to develop 100 per cent torque with 100 per cent magnetic flux, that is, at 100 per cent voltage. It is further assumed that under this condition the rotor slip, that is, the drop in speed from synchronous speed, is 4 per cent. The rotor copper loss, that is, the rotor current squared times the rotor resistance, equals 40 watts. If now the magnetic flux is reduced to per cent, then to produce the same torque the rotor current must be doubled, or increased to 600 amperes. Six hundred squared times .OoUee equals 160 watts. Asthe slip is proportional to the rotor loss the slip is now only 16 per cent, but the rotor current has been doubled, i. c., is twice its normal rated value, and the motor would rapidly overheat. In other words, to increase the rotor loss, viz., rotor current, of a low resistance rotor sufiiciently to reduce the motor speed appreciably requires such a large increase in the stator current to produce the required rotor loss that the motor, via, both rotor and stator, overheats. flux reduction to /3 of its normal value (even if the motor could be operated under such a condition) would require 3 times normal current or 9 times normal heating to produce even 36 per cent reduction in speed which would cause a very rapid overheating of the rotor.

If, however, the rotor resistance is increased to .01 ohm, for example, and the rating of the motorthat is, the load-reduced to /3 of its normal value, then the results will be as shown in Table II.

Table II Rotor Rc- Rotor HR 120- Flux, Percent sistance, Current, tor Loss, 1, t

hms Amperes Watts 0mm Here it is seen that a reduction of the magnetic flux to A? will produce a reduction in speed of 90 per cent, or reduce the motor speed to 10 per cent of synchronous speed, with a current of only 300 amperes, or its normal full loadrating. If normal ventilation can :be maintained at this reduced speed then the motor will not overheat. Such a constant torque load line is shown in Fig. 4 by As shown in Table I, a

dash line, 0. As seen in the curve, the speed of the motor is stable at all speed settings, and the maximum speed or the motor at its torque rating is up near the synchronous speed.

As shown in Tables I and II the necessary rotor loss to produce a required slip, that is, a desired reduction in speed, can he obtained with less current and therefore less heating of the stator winding as the rotor resistance is increased. It is also seen that as the slip is increased, and the motor speed reduced, the current required to pro duce a given torque increases. I have found that by increasing the rotor resistance sufficiently, a torque of the order of magnitude of /3 normal rated torque (i. e. about 30% to 50%) for a given motor size can he maintained at all speeds from maximum down to standstill without exceeding the normal rated current for the particular motor size. With such a combination or motor rating and rotor resistance it is possible to adjust the motor speed over the entire speed range, from a maximum speed. of within about 10 per cent of synchronous speed down to standstill, simply by adjustment of the line voltage applied to the motor, and without exceeding the normal rated current for the motor at any speed setting even when driving a constant torque load.

The value of resistance for the rotor which will provide the last above-described operation in accordance with this invention, may conveniently be determined experimentally by first selecting, for example, an induction motor of the squirrel cage type having a standard rotor, which will he of low resistance. Such standard motor will have known rated full load torque and full load current. A suitable ammeter is connected in series with. the stator winding and suitable meai s for decreasing the lin voltage should he provided. The rotor is then removed from the motor, placed in a lathe, and the end rings of the squirrel cage turned down slightly in order to increase the effective resistance. After re-assembling the motor, a load is applied to the motor which is approximately one-third (say 39% to 58%) of the full rated torque of the motor, and the line voltage is decreased until the motor slows to a standstill or to the desired minimum speed, while the animeter reading is observed. Finally, that value or motor resistance is selected at which just at standstill, or at the selected minimum speed, the motor current does not exceed a safe value. A safe value is that current in excess of which over heating results. If When this is done the maximum speed is too low, then a motor of larger size should be selected and the foregoing procedure repeated.

By way of example, it may be stated that by following the above mentioned procedure, a rotor resistance of oetween approximately 5 to 18 times that of a standard induction motor having loctween 3% and 4% slip, has been found to be satisfactory in providing speed control according to the invention. Those skilled in the art will appreciate that the rotor resistance here provided is of a different order of magnitude from that heretofore employed in induction motors. The range oi": resistanc above mentioned is, from a practical point of view, not as great as at first appears, because it includes both slip ring and squirrel cage types, the resistance of the former being normally about twice that of the latter. In other words, an average value of motor resistance for a slip ring motor according to the invention might he, say, 7 times the normal resistance heretofore employed; whereas for a squirrel cage motor it might be, say, 14 times, at a load or torque of, say, normal.

Load curves B in Figs. '2, 3 and 4 follow a square law, typical of loads such as fans and centrifugal pumps. These load curves cut the motor torque curves at much wider angles than the constant torque load lines A and C, and intersect the motor torque lines at only one point and on the stable part of the torque curves. In other words, this type of load characteristic curve nowhere coincides with the motor speed vs. torque curve, and intersects it at one point only. Stable operation with such loads can, therefore, be obtained at all speeds, with motor voltage control, with littl if any increase in rotor resistance. Also, because of the low torque required at the lower speeds, little decrease in motor rating is required to keep the motor from overheating.

The speed regulation of a motor with a high resistance rotor would ordinarily be very poor with change in load, namely, at any given setting a change in load would cause a Wide chang in speed. By the present invention, including an automatic control system, this difficulty is obviated, with the result that the motor speed can be held practically constant at each speed setting, independently of the load.

A simple form of the invention providing automatic speed control is illustrated in 5. The control system here illustrated will automatically control the speed of an induction motor at any desired value within limits of operation later referred to. The motor stator windings 2b, 2c are, as in Fig. 1, connected in series with the A. C. windings of similar reactors 5a, 5b, 5c, respectively. As before, the power line terminals are represented at Ma, i212, and l2c, respectively. The rotor i of the motor is not shown in detail, but may be assumed to be of the slip ring or squirrel cage type and of sufficiently high re--- sistance for use with the present control system, as explained in connection with Figs. 2, 3 and 4. The shaft of the motor is represented by a dash line 13 to which is mechanically coupled a centrifugal governor switch which may be of any suitable type well known in the art. Such switches have heretofore been employed to eiiect a change in resistance of the rotor at a certain speed, or to efiect other suitable circuit connections to provide sufiicient torque at starting and then to operate automatically at a predetermined speed to effect full-speed connections. A switch or" this type may be adjusted to operate at any desired speed within quite a wide range, and is here utilized to maintain the motor at the speed for which the switch has been pro-set, and to maintain that speed for all loads at which the motor is capable of operating.

Connected in series with the switch it are a resistor l8 and the secondary 2B of a power transformer l9. A condenser ll is connected across switch 16. Between the center tap or" transformer winding 20 and one terminal oi condenser ll is connected the primary of a coupling trans former 14, as shown. The secondary winding $5 of this transformer is connected in the control circuit of a full wave grid-controlled gas-filled tube, or Thyratron, rectifier which comprises rectifier tubes 8| and 9!, the respective grids 4'2 and d3 of which are connected to the two terminals of winding 15. The center tap of winding 15 is connected to the cathodes 44 and 45 of tubes 8! and ill in parallel.

lhe output circuit of this rectifier includes a power transformer it having a secondary windsuch as to furnish to each of 9 ing 2 I, the two terminals of which are connected respectively to the two plates 45 and 47 of tubes 8i, 9|, as is the practice in. the art. The direct current coils 3a, 3b and 3c of the reactors 5a, 5b and 50, respectively, are connected in series between the center tap of secondary winding 2| and the cathodes id and 35. Thus, alternating current power applied to primary winding as of transformer it is rectified by the rectifier 8!, 9i and the resulting direct or unidirectional current flows through direct current coils 3a, 3b and 3c. The value of this direct current, via, whether it be a maximum or a minimum, is automatically controlled in accordance with the motor speed, and thus controls the motor speed, as follows: When the switch it is closed the resistance in is connected across the secondary 2c of transformer 59. Under these conditions the alternating current (A. C.) voltage applied to the grids Q2 and is of tubes 8!, ill, is in phase with the voltage applied to the plates it, ll of these tubes through transformer id, and a maximum value of direct current flows between terminals 6 and l of the D. C. control circuit. This reduces the reactance of the reactors 5a, 5b and 50 to a minimum, resulting in maximum line Voltage applied to stator windings 2a, 2b and which tends to increase the speed of the motor to its maximum. When the speed of the motor increases the centrifugal governor switch it opens which connects condenser i? and resistance is in series across transformer winding 2%. This shifts the phase of the voltage on the grids s2, 43 with respect to the voltage on the plates 46 and 41 substantially 189,

which efiectively opens the plate circuits of tubes 6i and iii so that substantially no direct current flows through terminals 6 and 1 or through coils 3b and 3c. The resulting decrease in the D. C. fields of these last-mentioned D. C. coils increases the reactance of the reactors to a maximum, reducing the applied line voltage to the stator windings 2a, 2b and 2c and slowing down the motor. When the motor slows down to a predetermined speed for which the switch it is adjusted it closes again and the above-described operation is automatically repeated.

The foregoing control operation obviously causes a slight speed fluctuation of the motor the magnitude of which depends largely upon the design of the governor. However, there are available switches suinciently sensitive to change in speed that the resulting fluctuation in speed is not detrimental for many purposes. The practical result of such a system is to give to an induction motor having otherwise poor regulation the desirable characteristics of a shunt wound directcurrent motor.

To assist those skilled in the art to carry out this invention it is suggested, purely by Way of example, that resistor i8 may have a value of 1% ohms and that condenser ll be of approximately 2 rnicrofarads, if a 60 cycle power source is employed. Transformers [9 and it may be the grids 42 and 43 a potential of about 5 volts. It should be pointed out iii-connection with the system of Fig. 5 as with all the other control systems herein described, that the power furnished to and handled by the control system per se is comparatively small and, in general, may be the same regardless of the load on the motor because the motor windings are independently connected to the line ter minals i2a, i222 and Ho. Thus the motor current does not pass through any electronic tubes,

switches, or movable elements, but only through reactors 5a, 5b and 50 which should be of a rating suitable to handle the motor current.

The control system of Fig. 6 is, like that of Fig. 5, an off and on control, but includes certain advantages and refinements which will be described below. The advantage which is first apparent is that the centrifugal governor switch of Fig. 5 is replaced by an electronic system which operates in response to variation in voltage generated by a small pilot alternator preferably of simple and reliable type. The frame and shaft of this alternator may be fastened directly to the motor frame and shaft, respectively.

The alternator referred to, which is shown at the left of the diagram comprising Fig. 6 is designated by the reference character and as shown is mechanically coupled to the motor shaft i3. A direct-current generator could be substituted for this alternator together with the rectifier connected to its output as shown in Fig. 6a, but it has been found in practice that an alternator is more reliable because it may be of the inductor type which eliminates commutators, brushes and slip rings, together with their inherent disadvantages. However, the D. C. machine inherently has the advantage of being polarity sensitive, via, the polarity of its output voltage changes with direction of rotation, which is not true of the alternator. Such alternator may be of the single phase inductor type provided with enough poles to generate a useful frequency at low speeds. A design found to be suitable for this purpose generates a 210 cycle voltage at 1800 E. P. M., although other suitable designs could be employed.

The output terminals til, 5 of alternator lil are connected to the two plates of a rectifier tube 22. This tube might be of the type 6X5, for example, which is of the indirectly heated cathode type. This tube, having two plates and thus providing full-wave rectification, requires a center tap or neutral point of connection to the A. C. power supply. I-lere such a point is represented by center tap 25 on the winding of the alternator. It could as well be the center tap of a resistor or of an inductance or transformer Winding effectively connected across the output terminals. (The latter scheme is employed in the system of Fig. 8.) Between center tap 2s and the cathode 5! or the rectifier are connected a condenser C1 and resistor R1 in parallel. By way of example, condenser C1 might have'a value of l microiarad and resistor R1 a value of 100,000 ohms. Thus there is developed across resistor R1 a D. C. potential (positive at the top of the resistor and negative at the bottom, as seen in the drawing) which varies substantially in proportion to the speed of shaft it: An amplifier tube preferably of the high mu type such as type 6P5, is controlled by the fluctuation of potential developed across resistor R1. For this purpose the grid of tube 23 is connected to the positive terminal of resistor R1, the negative terminal of which is connected to the cathode $3 of tube 23 through a potentiometer R5 which is connected in shunt to a part of resistor R4. The output or plate aircult of tube 23 includes resistor R2 and a portion of resistor R4, as shown. Resistor R2 is a coupling resistor whereas resistor R4 is a voltage divider, suitable direct-current potential for which is derived from rectifier 25, power transformer 25 having a center-tapped secondary 2i, and filter condenser C2. Such a source of direct current potential is common in the art. Thus from voltage divider R4 are derived the necessary operating potentials for the plate and grid electrodes of 11 tube ,23. Obviously any other suitable potential source could be substituted.

A second amplifier tube 2c is coupled to the output of tube 23. Tube 24 may preferably be of a type havin a fairly high plate current such as type 2A3. Connected in the plate circuit of tube 24 is a suitable power limiting resistor R6 and a coupling resistor R3 which, for example, may be of 5,000 ohms. A battery B1 (which may be any other suitable source of D. C. potential) is connected with its positive terminal to the lower terminal or resistor R3 and its negative terminal to the cathode 5b of tube 24. To the output cir-- cuit of tube 24 is coupled a Thyratron rectifier similar to that shown in Fig. 5 where, as in the other figures, like components are designated by like reference characters. However, in the system of Fig. 6 the Thyratron tubes 8t and 9i are controlled by biasing the grids l2, c3 thereof with a direct current, potential instead of an alternating potential as in Fig. 5. Hence a battery or other suitable source of direct current B2 is connected with its negative terminal to resistor R3 and its positive terminal to the cathodes 5d and 45 of tubes 8! and ti, respectively. this circuit the grids t? and 43 are connected to the positive terminal of resistor R3.

The operation of the system of 6 is as follows: With the motor stopped and no potential developed across resistor R1, as resulting from U the generation of a potential by alternator the tube 23 may be assumed to be biased nega tively by reason of the adjustment of slider on speed control potentiometer R5 toward the negative end thereof. Under these conditions no plate current flows through resistor R2 and hence there is zero grid bias on the grid of tube 24 which makes that tube conductive, via, current flows through the plate circuit and hence through resistor R3. Current flowing through resistor R3 develops a positive potential at terminal 52 which in turn is connected to the grids til and 43 of Thyratron tubes 8| and &I, respectively. A positive bias on grids t2 and 43 causes these tubes to conduct which, in turn, as explained in corn nection with the system of Fig. 5, decreases equally the reactances of the reactors assumed to be connected to terminals 5 and l, as in Fig. 5, and the motor starts. As the motor increases in speed, a voltage is developed across terminals 49 and 50 of alternator (it, which develops a potential across resistor R1 biasing tube as positive, causing current to flow through R; which biases the grid of tube 24 negative. This cuts off the current through resistor R3, permitting the bias battery B2 to bias grids 42 and 43 strongly negative, shutting off the direct current to terminals 6 and 1. As before, the result of cutting off the direct current in the reactor coils decreases the motor speed. From the foregoing it, will be ob served that the motor speed will thus be maintained effectively at a rate determined by the position of adjustable contact 30 on potentiometer Rs. Hence by means of the system of the invention illustrated in Fig. 6, the motor may be started, stopped and its speed changed during operation by control of a single adjustable element and the selected speed will be automatically maintained substantially at the predetermined rate.

In the control system of Fig. 7 the generator 50 and rectifier 22 are represented as in Fig. 6. Also as in Fig. 6 the input circuit of vacuum tube 23 includes resistor R1 across which is developed a direct current voltage substantially proportional to the speed of generator 40 which, as before, is

To complete driven directly by the motor of which the speed to be controlled. The A. C. generator and rectifier may be replaced, as shown in Fig. Go, by a D. 0. generator the output voltage of which is impressed across resistor R1. The voltage across resistor R1 may for convenience be called the control voltage or the speed voltage. By way of example, the resistance of resistor R1 may be approximately 100,000 ohms and the capacity of condenser C1 connected across it may be approximately 1 microfarad. In any event, the values of these two components should preferably be proportioned to provide the required stability, via, freedom from hunting of the motor being controlled. The full Wave rectifier tube furnishes plate voltage to tube 23 through coupler resistor R2 which may, for example be of megohm and also furnishes a direct current voltage across resistor R4 which functions as a volttage divider. A part of resistor R4 is connected in the plate circuit of tube 23 in series with resistor R2 as in the case of Fig. 6. By connecting the cathode 53 to the point 0 on resistor R4 the portion of resistor R4 to the left of 0 will provide a negative bias to the grid of tube 23 when contactor or slider makes connection with such portion, as is shown in the drawing, because the two voltages are effectively in series. On the other hand when slider 30 is moved to the right of point t, a positive bias of variable magnitude will be impressed on the grid. Hence, as the voltage developed across R1 impresses a negative bias on the grid 54 of tube 23 substantially proportional to the speed of generator 40, and the reference voltage derived from the position of slider 30 may be varied from a negative value which will aid the voltage across R1 to any desired positive value which will buck the voltage across R1, the current flowing in the plate circuit of tube 23 at any given speed of the generator may be predetermined by adjustment of slider 30. From this it will be seen that the eifective bias voltage impressed on grid 54 comprises the algebraic sum of the control voltage across resistor R1 and the reference voltage across that portion of resistor R between the point 0 and the slider 30. Thus, this resultant voltage applied as a bias to tube 23 may be either positive or negative depending upon the relative magnitudes of the component voltages. The position of the point 0 on resistor Rd will in any specific case obviously be determined by the characteristics of the tube 23, and by the type and degree of control desired.

The input circuit of vacuum tube 24, which, for example, may be similar to tube 24 of Fig. ii, is coupled to the plate circuit of tube 23, but the connections are reversed in respect to customary procedure, and in respect to the arrangement of Fig. 6. It will be noted that the cathode 55 of tube 24 is connected to the plate 5'! of tube 23 and to the negative end of resistor 112, whereas in Fig. 6 the cathode 55 of tube 24 is connected to the positive terminal of resistor R2. Likewise, in Fig. '7, the grid 53 of tube 24 is connected to a point 55 on resistor R4 which is somewhat more negative than the positive terminal of resistor R2 instead of being connected to the negative end of resistor R2 as in Fig. 6. The purpose of this reversal in coupling connections is explained below in connection with the operation of this systern.

The plate circuit of vacuum tube 24 includes a coil 35 comprising the direct-current control coil of a saturable core reactor 33 which may obviously be smaller tha those in the motor The phase of the grids power circuit. In this arragement a third fullwave rectifier tube 3| fed by a power transformer 32 is connected to the plate circuit, of tube 24 and thus replaces the equivalent battery B1 of Fig. 6. The alternating-current Winding 34 of reactor 33 is connected in a series circuit comprising also resistor 37 and the secondary winding 59 of a transformer 35. The primary winding 86 of transformer 36 is assumed to be connected, as are the other primary windings of transformers 26, 32 and it to a suitable alternating power source, say of 60 cycles. In this connection it may be mentioned that transformers l and 36 should be connected to the same power source. The primary winding 39 of transformer 38 is connected between the center tap of secondary 59 and a point 6! in the series circuit between coil 34 and resistor 31. The rectifier comprising Thyratron tubes 8i, 9! is a full wave rectifier, the plates 4%, 4i of these tubes being connected to the two terminals of the secondary 2| of transformer I 0, and the grids 42, 43 being connected to the two terminals of the secondary winding 62 of the transformer 38. Cathodes 44 and 45 of tubes 8i, 9| are connected together and to the center tap of secondary winding 62. Thus the center tap of wind ing 2| comprises a negative terminal, and the center tap of Winding 62 comprises a positive terminal, and to these, through terminals 6 and i are connected the direct current control coils of the saturable core reactors as in the preceding figures.

The operation of the control system of Fig. 7 is an follows: Assuming that the motor is stopped, and hence generator 40 is not generating, the motor may be started by sliding control 353 to the right of point ii on resistor R4. This will bias grid 54 of tube 23 positive causing plate current to flow through resistor R2 which in turn will bias the grid 58 of tube 24 positive with respect to cathode 56, causing plate current to flow through direct-current control coil 35. The reactance of reactor 33 is thus reduced to a value effectively less than the resistance of resistor 31 which in turn causes the A. C. voltage applied to grids 42 and 43 of Thyratron tubes 8|, 9!, respectively, to be substantially in phase with the A. C. voltage impressed on plates 46 and 41 of those tubes, with the result that a fairly large direct current flows through terminals 6 and 1, and through the direct current control coils 3a, 3b and 3c of the motor control reactors a, 5b and 50. As previously indicated in connection with Fig. 1, this reduces the eifective reactance of these reactors equally and permits a balanced starting current to flow through the motor. The result of this application of a balanced voltage to the stator windings of the motor is to produce a single revolving magnetic field which rotates in one direction only.

When the motor has reached, and, for example, just exceeded a speed which corresponds to the setting of slider 3!] on resistor R4, the positive bias theretofore impressed on grid 54 of tube 23 will be opposed and slightly exceeded by the voltage drop across resistor R1, causing tube 23 to be biased slightly negative. This reduces the current in resistor R2 and correspondingly decreases the positive bias on grid 58 of tube 24, or causes the bias on that grid to become more negative depending upon the degree of change, thus decreasing the current in control coil 35 of reactor 33. This increases the effective reactance of reactor 33 so as to be, say, slightly higher in its effect than the resistance of resistor 31. 42, 43 with respect to the plates 46, 41 of tubes 8|, 9|, respectively, will thereby be shifted in an out-of-phase direction a corresponding degree, with the result that the current through terminals 6 and i will be decreased and the motor speed correspondingly decreased until the entire system is in balance.

A control system as shown in Fig. 7 is quite sensitive to changes of motor speed and will, therefore, maintain the motor speed at a quite constant value with completely smooth operation. However, shouldthe automatic regulation of speed be inadequate in any given instance, closer regulation can be attained by substituting an additional stage or stages of amplification for the single amplifier stage comprising tube 23 and its associated circuits.

From the foregoing it will be understood that the speed at which the system of Fig. 7 maintains the motor is determined by the relation of the potential drop across resistor R1 to the potential drop across that portion of resistor R4 between point 0 and the position of adjustable contact or slider 3i (assuming that slider 323 is to the right of point 0). When slider 30 is in the mentioned position and it is desired to stop the motor, it may be necessary to move the slider to the left of point 0 on resistor R4. In any event by so doing the grid of tube 23 will be biased well beyond cut-off with the result that the power to the motor will be immediately decreased substantially to zero, thus quickly stopping the motor. By reversing the coupling between tubes 23 and 24, as above mentioned, at least one ad" vantage is introduced over the operation of Fig. 6, viz., if for any reason the plate current of tube 23 is interrupted, as by failure of the tube, the motor will stop, whereas in the arrangement of Fig. 6 the motor will run at full speed.

The control system illustrated in Fig. 8 mm vides not only complete control of the speed of an induction motor and automatic maintenance thereof at any preselected value, as in the system of Fig. 7, but also permits reversal of the motor and similar speed control in the reverse direction. Although any convenient form of control device could as well be provided, the one illustrated in Fig. 8 of the rotary rheostat type actuated by a single knob has been found satisfactory. At a zero or balance point intermediate the two extremes of rotation of this knob the motor will be at a standstill, and as the knob is rotated in one direction the motor will start and gain speed until maximum speed is reached at the extreme position of the knob in the direc tion in which it was first moved. As the knob is rotated back toward the zero position the motor will slow down to a stop, and then if the knob is rotated in the same reverse direction away from the zero position the motor will start in the reverse direction and gain speed until full speed is reached at the extreme position of the knob in the latter direction.

The system of Fig. 8 comprises essentially two control channels each being similar to the system of Fig. 7. These two channels are connected together in such manner as to cooperate to provide additional advantages, including: (1) the control of the motor speed and direction of rotation by a single remote control means, (2) the connection of two sets of variable reactors between the motor and the line in reverse order so as to effect control of direction as well as speed of the motor, and (.3) an arrangement whereby at the "zerc position of the control at which the two control channels are effectively in balance in respect to the motor, the motor will stop without opening the line to the motor, because, as explained at the outset, this invention permits complete control of the motor without employ ing contacts or movable elements in the motor power circuit. It will be evident that although control of the speed and direction of rotation by a single control as here shown may be preferred, the invention is not limited to such unicontrol.

The control channel at the upper part of Fig. 3 is represented in a manner generally similar to that of Fig. 7, corresponding components being designated by corresponding reference characters. The control channel at the bottom of Fig. 8 will be seen to be similar to that of the upper channel, and circuit elements or components which correspond thereto are designated by the same reference characters plus 100. Therefore, the operation of each of the two channels individually is the same as that described in connection with Fig. 7, although as here combined, the two channels provide a system having additional features of operation. Both channels function at all times, but in a different manner except when the motor is held at standstill, that is, while one channel maintains the magnitude of A. C. current through one set of A. C. windings at substantially zero, the other channel maintains the A. C. current through the other set of A. C. windings at the magnitude required to operate the motor at the preselected speed in the preselected direction.

Referring to Fig. 8, a generator All is mechanically coupled to motor shaft i3 and the output of this generator is electrically coupled to two rectifier tubes 22 and H2 through transformers 4i and Hi, respectively. The center taps 29a and HM on the secondary windings of these two transformers correspond functionally to the center tap 29 on the generator winding of Fig. '7. To control the direction of rotation and speed of the motor a single control knob 92 is provided which functions as a unitary control for sliders 3e and I30 on resistors R4 and R104, respectively. It will be noted that the terminals of these two resistors are connected respectively to the positive and negative terminals of the same direct current voltage supply comprising rectifier and its associated components. A separate voltage supply could be connected to each resistor, but it is preferable to employ a single supply, because it would be diincult to maintain two voltage sources at exact equality.

To control the alternating-current power to the motor, two reactors of the saturalole core typ are shown connected in series with each phase of the motor and the line. In order to effect reversal of the direction of rotation of the motor the A. C. windings of one set of reactors are connected in the reverse order with respect to the other set. This relative order of connection can conveniently b seen at the power line terminals which are designated l2cb, [Zoe and lZaa to indicate the order of connection of the reactor windings. In a three-phase system, as here illustrated by way of example, two of the phases are connected to the line in reverse order with respect to each other, but the third phase connection, l2aa, is necessarily the same. In a two-phase system this phase would be omitted.

The operation of the control system of Fig. 3, in some respects is similar to that of Fig. '7, but differs in other respects. When the sliders and 130 are in the position shown in the drawing the grids 54 and i54 of tubes 23 and I23 are equally biased slightly negative, 1. e., are biased sufficiently to cut off the currents in the plate circuits. When knob 92 is rotated so that, for example, slider 35! is in contact with tap o, the grid 54 or tube 23 is at the same potential as the cathode 53 thereof, and plate current will begin to flow through that tube and through resistor R2. at this adjustment of sliders 3i and its, tube 123 will be biased beyond cut-off. The effective reactance of reactors 5a, 5b and 50 will then be less than that of reactors W502, H352) and I550, causing the motor to rotate in the corresponding direction. As a practical matter, the current through the second set of reactors ih'aa, Hill?) and liic will then be substantially zero and will remain so at all settings of slider 30 between point e and the positive terminal of resistor its-- which corresponds to the entire speed range of the motor in that direction. Similarly, when slider I33 is connected to tap point lot the motor will start (or tend to start, depending upon the load) in the reverse direction, because the effectiv reactance of the second set of reactors Nita, H351) and the will be less than that of the first set lib and he, and in fact, while the slider 13b is anywhere between point lit and the posi tive terminal. or resistor R104, the upper channel will be biased beyond cut-oft so as to substantialiy prevent current flowing to the motor wind ings through reactors to, 5b and 50 while the speed is under the control of reactors H5511, i951) and H150. Thus the eiiective reactance of one set of reactors is maintained at substantially maximum while the effective reactance of the other set of reactors is being varied to obtain speed control.

It may he noted that there are two alternatingcurrent reactor windings connected in series between each to phases of the line. However, the reactors are so constructed and connected together that the eiiective reactance of this interphase connection is so high that no appreciable short-circuit current flows. The only condition under which there would tend to be a short-circuit would be in the event that th reactance of both sets of reactors were simultaneously reduced by the direct-current control circuits. However, in the described system of this invention there no possible setting of the controls by which this could occur. As in Fig. 7, the arrangement of Fig. 8 is not limited to the type of reactors illustrated, for other equivalent types could b employed. v

Referring finally to Fig. 9, there is illustrated a simplified control system by which the speed and direction of rotation of an induction motor may be manually controlled, but without the automatic feature included in the system of Fig. 8. The general arrangement of the system of Fig. 9 corresponds to that of Fig. 1 in much the same manner that the system of Fig. 8 corresponds to that of Fig. 7. It is intended that the control system to the left of terminals 6, 1, W8 and Id"! of Fig. 8 be replaced by that to the left of the same terminals in Fig. 9. This system comprises a double rheostat operated by a uni control knob 92 and including resistors II and i H which correspond functionally to resistors R4 and R104 of Fig. 8. The two resistors II and III are connected to terminal iii of any suitable direct current source. The two terminals 1 and H11 are connected to the other terminal lol of the directcurrent source. The sliders 30 and of the 17 rheostat are connected respectively to the two terminals 6 and I06.

It will be recalled in connection with the description of the system of Fig. 8 that to achieve satisfactory operation it is desirable that on set of motor control reactors be maintained at maximum efiective reactance while the effective reactance of the other set of reactors is being varied to obtain speed control. This result is effected in the arrangement of Fig. 9 by providing two conductive or very low resistance segments or sectors 62 and I62 on which sliders 30 and 130, respectively, ride. These sectors are connected to the resistors l and HI which are connected to the terminal SI and thus provide a uniform maximum reactance in one set of reactors while the reactance of the outer set is being varied to control the speed in the chosen direction of rotation of the motor. Differently stated, the speed of the motor is increased by decreasing the resistance in series with one set of D. C. coils and increasing the current in this set of coils whilst automatically holding at a minimum value, or decreasing, the current in the other set of D. C.

coils. It is evident that other equivalent types of voltage control means could be substituted for the double rheostat of rotary type here described by way of example.

What is claimed is:

1. In a system for controlling the speed and direction of rotation of a multiphase A. C. induction motor having a winding for each phase, a first set of saturable core reactors each having an A. C. winding and a D. C. coil associated therewith, the A. C. winding of each reactor being connected between one phase of the line and one of said motor windings; respectively, at least one D. C. source, and a variable resistor, first circuit means connecting the D. C. coils of said reactors in series with said D. C. source and said resistor, a second set of saturable core reactors similar to those of said first set, the A. C. winding of each of said second set of reactors being also connected between one phase of the line and one of said motor windings, respectively, the A. C. windings of said second set being connected to said phases in reverse order with respect to those of said first set, a second variable resistor, second circuit means connecting the D. C. coils of said second set of reactors in series with said D. C. source and said second resistor, and unicontrol means for simultaneously varying said resistors so as to increase the current in the first set of D. C. coils whilst automatically decreasing the current in the second set of D. C. versa.

2. A system according to claim 1, in which said unicontrol means includes means for simultaneously varying said resistors so as to vary the current in the first set of D. C. coils whilst automatically maintaining the current in the second set of D. C. coils below a substantially fixed predetermined value, and vice-versa.

3. In a system for controlling the speed and direction of rotation of a multiphase A. C. induction motor having a winding for each phase, a first set of saturable core reactors each having an A. C. winding and a D. C. coil associated therewith, the 'A. C. winding of each reactor being connected between one phase of the line and one of said motor windings, respectively, a second set of saturable core reactors similar to those of said first set each having an A. C. winding and a D. C. coil associated therewith, the A. C. windding of each of said second set of reactors being coils, and vice- 'mally having such a high duction'motor having or said second'set of reactors nected between one phase of the also connected between one phase Of the line and one of said motor windings, respectively, the A. C. windings of said second set being connected to said phases in reverse order with respect to those of said first set, D. C. source means for energizing said D. C. coils, first and second variable resistors, and circuit means interconnecting said D. C. coils, said source means and the respective resistors such that adjustment of said resistors varies the D. C. current in said sets of coils selectively, and unicontrol means for simultaneously adjusting said resistors so as to vary the current in the first set of D. C. coils whilst automatically maintaining the current in the second set of D. C. coils below a predetermined fixed value, and vice-versa.

4. A circuit for controlling the direction and speed of rotation of a multi-phase induction motor, comprising a multi-phase power circuit for the motor, a plurality of saturable core reactors arranged in two sets each having an alternating-current winding for each phase of the power circuit, direct-current windings electromagnetioally associated with said alternatingcurrent windings, the alternating-current windings being connected one in each line wire of the power circuit in series with the motor with the alternating-current windings of one set of reactors connected to produce a difierent phase sequence from the windings of the other set of reactors, the alternating-current windings norimpedance as to prevent any material application of voltage to the motor, and control circuit'means operable selectively and variably to" energize the direct-current windings to govern the direction and speed of rotation of the motor 'by control of the magni tudeand phase sequence of the voltage applied to the motor, said means including means for producing a voltage corresponding to the desired speed of the motor; and means for stabilizing operation of the motor at the speed desired. 5.'In a system for-controlling the speed and direction of rotation of'a multi-phase A. C. in-

a winding 'for each phase, a first set of'saturable core reactors, each reactor having an A. C. winding and a D. C. coil, the A. C. winding of each reactor being connected between one phase of the line and one of said motor windings, respectively, D. C. source means connectible to said coils, a first D. Cpcontrol device coupled tosaid D. C. source means and adjustable in a first direction to increase the current through the first set of D. C. coils, a second set of saturable core reactors similar to those of said first set, the A. C. winding of each being also conline and one of said motor windings, respectively, said second set of A. C. windings being connected to said phases in reverse order with respect to those of said first set, asecond D. C. control device coupled to said D. C; source means and adjustable simultaneously with adjustment of'said first device in said first direction to decrease the current through the second set of D. C. coils, each of said D. C. control devices being provided with means which maintains the current through one set of said D. C. coils substantially constant at a minimum value whilst the current through'the other set of said D'. C. coils is adjusted over a speed-control range, and unicontrol means for simultaneously adjusting said D. C. control devices corre sponding to motor rotation in either direction.

6. Ina system for controlling the speed'and saidiD. C; 'source means andea'djustablelzin aifirst direction 'to increase the current .Ithrough ithe first set of. D. C.=:coi'ls,: a-second:' setiof saturable iCOI'BTBaClSOrSi similar to'those of said-first set, .the 5A.:1C. windings of each'oi said second setzof rerthet'lineiand one iof: said" motor 'Yvvindingsprespectively, said second set 'of ArC-s windings bet ing cormectedto saidlphasesinreverse' orderwith rrespect to; those tofsaid' first set a'second DLC. #:ccntroldevice coupled to'said 'D C. sourcemeans --and"adjustable in said firstdirectionto decrease the current through thet second setdf? D; Crcoils, eeach: ofesaid D. :C. control ldevicesarcomprisingn similar rheostat'havingaa:highrresistance element 'andea-Jowrre'sistance element connected iin series, two adjustable contact members zone for each of said serieseconnected elementspa plurality of ccouplingi'meansr connected rrespect-ively :to each of said: contactmembersnmdito terminals on said "resistance.elementsaandi'operativeItoconnect the 1-D; C. coils: ofceach eset respectively-'1 to said- D5 C. source v:means, rand mini-menthol 1 means coupling both; ofcsaidccontact imembers 'soithat. only one of "said: members is inicontact withzadow-resistance elementat'; airtime; wherebyl theFD. .C. currentin theifirsttset :of: D. C: rco'ils. is zmaintamed substan- :tially "'constantcatraz minimumwaluewhilst the 'D; Crrcurrent imtheesecond set of D.i' C; coils is vvaried overva. speedecontrolrrange: from or to a :minimum value; and vicewersa.

v7. :I-n: la system:.=.according svto -claim'f3':wherein said D: C'. sourcezmeans includes a source ot'DIC. voltage? the magnitude :01 w-hichf is' automatically rproportional ii'substantia'lly :to thewsp'ee'd of said motor, electronicicoupling lmeanscconnected between :each said-variableresistor 'arid 'the set: of -13. C.: coils withwhich: iti is 'associatedgand connections 5 from *said rresistors zto' said *electronic vcoupl-ing means coperativerto 'smaintain asubstantiallycminimum vatuerof .D; C:. current in one a set of DJ Csc'oi-lst whilstilthe D11 Clrcurrent' in the 'other-setrofLD.C:coils: isrvariedaoveria range ahetweencatgminimum andza greater value.

=8. A system:accordingtto. claim 3; wherein said ..D.=,C.-::source means includeswai' source of Dr'C. i voltage", theimagnitudecof which: is automatically proportional csubstantially L130 the speed of I. said 4 motor; a coupling system for: coupling-e each-said *resistorrto the set? of1D.: C. -::coi'ls with whichi ittis ::=associate d,'- saidrresistors iibeingrarranged so .that 5 said; =unicontrolrmeansiris :adiustahlertoca position 3,13 'WhichTSflidfiSfitS :of reactors areubalanced zito 1stopcrotationcofssaid motor; and connections in said coupling :asystemsi providing an-zradjustable *speed'. range: for :saidrmotorriin Ffiithfll. direction :of rrotation when i'said unicontroliimeans is moved .inxeither directionrrespectively, fromssaid balancedrrposition.

:9. In a;system%tor -controlling the speed -of-- an C.s-induction -motor operable "tr-omen A. .C. =source, means i for .generating .a D. 5C. control voltage .which .is substantially.proportional to .the speed of said motor,.-.means responsive to variation .in. D. C. voltage .for controlling the A. .C. .voltage .applied..fnom..said.scurce .to .said motor, and a :coupling system connected between said .D. C." voltagergenerating means :ands said;:re

sponsive means including, a first: vacuum tube amplifier having grid and plate: circuits; an 7 adjustable :biasing voltage effectively connectediin of-said two voltages-being efiectiveto biasrsaid grid circuit, a secondvacuum tubeamplifier'having gridtand plate circuits;said-*first'tplate circuit being coupled to saidsecondsgrid circuit, arsaturable core reactor having all C. coil and an winding, said D. C; coilbeingwconnectediinrthe plate circuit :of saidseoond amplifier tube-1a first A. C. transformer: having a: primary-winding i connected to said'A; C. source and a' secondary winding which is connected: in a-seriescoircuit including said :A. C. wwinding and a resistor, ra xSBCOIl-d A. C. trans-former having a primary and a secondary, I said esecond' primary ebeing :connected betweenacenter tap on:said-: secondaryof said! first transformer and-.a point ins-saidrsenies circuit. between' said A. C. :coilrand said resistor,

a pair of gas-fllled;'gridecontrolled 'rectifierztubes coupled in full-waverrectifien relation-:withztheir "grids connected .170 =.the two-terminalscoir-the secondary of saidssecond: transformer, a,;third LA. C. transformer having?acprimaryiconnectedhto said A., C. source and .asecondary; provided with a midpoint and two terminals,--'saidl.last =named terminals being connected respectively etc the plates of said rectifier --.tubes, =a connection ;be-

tween the cathodes oflsaid rectifier tubesrandxthe midpoint of the secondaryaofisaid-second trans- I former, and connections. from said midpoints of said second and third'transf'ormers tot-said: means responsive to variationeinr D; .Cxcontrol voltage, whereby the speed:ofwsaid- -motorrrcorresponding toany 'preseiected value.- 20f: isaid biasing voltage will be maintained: substantially constant.

10. In asystemior controlling thexspeedcof an A. C; induction motoriopera-ble irom a an C.

source, means for generating'sal D.'C.rcontroi volt age which is substantially -=.=proportional to :the speed of saidrnotor; means responsive to variation in D. C. voltage iorcontrolling theA. C1 '-voltage applied from-saidzsourcetosaidinotor; and a coupling systemzconnected :between' said DJ'C. "voltage generating :means and said 'responsive 'means including, a-first'vacuum'tube. amplifier having grid and :plate-circuits, an adjustable biasing voltage eiiectively "connectedinseries with said control voltage," the resultant of: said "two voltages being 'effective'to bias'sald'grid circuit, a second vacuum'tube amplifier having-grid :and plate'circuits, af'souree of D.'C.' potential,

a'connection from the plate of said firstamplifier tube to a positive terminal of said source,

xaconnection from the rgridrof said second" amplifier tube to a terminalrof saidsourceamorernegaftivethan said positive terminal; and aoonnectionirom the-plate or said first amplifier tuherto the I cathode of said *secon'd'" amplifier ftube, ::a "saturable core*reactor -having a D. Cu. coil and an A. C. winding; saidDIX-coilbeingconnected iirtheplate circuitof'said second amplifier tube, a first A. Ctrans'former havinga' primary wind ing connected to :said sourcecand i a secondary winding which 'is connected in a }ZSEY-iGS" circuit including said A. ,C. windingwandva resistor, a

second -A. C. transfotmerqhaving a primary and a secondary, saidsecond primary beingconnected between a center ctap .on said secondary of .said first transformer'and a point in saidtseries circuit betweenisaid A. Cmcoilcandsaid resistor a'pair of gas-filled gridecontrolled rectifier tubes coupled iniull-wava rectifier; relationwith their ids connected to -to said D. C. source and to the two terminals of the secondary of said second transformer, a third A. C. transformer having a primary connected to said source and a secondary provided with a midpoint and two terminals, said last-named terminals being connected respectively to the plates of said rectifier tubes, a connection between the cathodes of said rectifier tubes and the midpoint of the secondary of said second transformer and connections from said midpoints of said second and third transformers to said to variation in D. C. control means responsive voltage, whereby the speed of said motor corresponding to any preselected value of said biasing voltage will be maintained substantially constant.

11. In a system for controlling the speed and direction of rotation of a multi-phase A. C.

induction motor having a winding for each phase, a first set of saturable core reactors, each reactor having an A. C. winding and a D. C. coil, the A. C. winding of each reactor being connected between one phase of the line and one of said motor windings, respectively, a D. C. source and a first D. C. control device adjustable in a first .direction to increase the current therethrough,

the D. C. coils of said reactors being connected said controldevice, a second set of saturable core reactors similar to those of said first set, the A. C. winding of each of said second set of reactors being also connected between one phase of the line and -one of said motor windings, respectively, said second set of A. C. windings being connected to said phases in reverse order with respect to those of said first set, a second D. C. control device adjustable in said first direction to decrease the current therethrough, the D. C. coils of said second set of reactors being connected to said D. C. source and to said second control device, each of said D. C. control devices beingprovided with means which maintains the current through one set of said D. C. coils substantially constant at a minimum value whilst the current through the other set of said D. C. coils is adjusted over a speed-control range, and unicontrol means for simultaneously adjusting said D. C. control devices corresponding to motor rotation in either direction.

12. In a system for controlling the speed and direction of rotation of a multi-phase A. C. induction motor having a winding for each phase, a first set of saturable core reactors, each reactor having an A. C. winding and a D. C. coil, the A. C. winding of each reactor being con- .nected between one phase of the line and one of said motor windings, respectively, a D. C. source and a first D. C. control device adjustable in a first direction to increase the current therethrough, the D. C. coils of said reactors being connected in series between said D. C. sourceQand said control device, a second set of saturable core reactors similar to those of said fiIStTfSGt, the A. C. winding of each of said second set of reactors being also connected between one phase or" the line and one of said motor windings, respectively, said second set of A. C. windings being connected to said phases in reverse. order with respect to those of said first set, a second D. C. control device adjustable in said first direction to decrease the current therethrough, the D. C. coils of said second set of reactors being connected in series between said D. C. source and said second control device, each of said D. C.

control devices comprising a similar rheostat having a high resistance element and a low reseries-connected elements, a connection from each of said contact members to one terminal of the series-connected D. C. coils of one set respectively, a connection from the other terminals of said D. C. coils to one terminal of said D. C. source, a connection from the terminals of both high resistance elements to the other terminal of said D. C. source, and uni-control means coupling both of said contact members so that only one of said members is in contact with a low-resistance element at a time, whereby the D. C. current in the first set of D. C. coils is maintained substantially constant at a minimum value whilst the D. C. current in the second set of D. C. coils is varied over a speed-control range from and to a minimum value, and vice versa.

13. In a system for preselecting any speed of an A. C. induction motor over a large range and automatically maintaining the selected speed with varying load, a source of A. C. power for said motor, said motor having as many windings as there are phases of said source, means for generating a speed-responsive voltage which is a function of the speed of said motor, speed-selectin means including a source of adjustable reference voltage connected in said system effectively in series with said speed-responsive Voltage so as to produce a resultant voltage comprising the algebraic sum of said speed-responsive voltage and said referenc voltage, a saturable core reactor having an A. C. coil connected be- .ween each phase of said source and a corresponding motor Winding, by which a balanced Voltage is applied to the motor so as to produce a single revolving magnetic field, said reactor having a D. C. winding associated therewith, and means continuously controlled by said resultant voltage so as automatically to adjust the current in said D. C. Winding and thereby the balanced A. C. voltage applied to said motor as a function of said resultant voltage to a value which maintains the preselected motor speed substantially constant.

14. In a system for preselecting any speed of an A. C. induction motor over a large range and automatically maintaining the selected speed 7 with varying load, a source of A. C. power for said motor, said motor having as many windings as there are phases of said source, means including a generator driven by said motor for generating voltag which is subspeed-selecting means including a source of adjustable D. C. reference voltage connected in said system eiTectively in series with said speed-responsive voltage so as to produce a resultant voltage comprising the algebraic sum of said speed-responsive voltage and said referencevoltage, and a saturable core reactor having an A. C. coil connected between each phase of said source and a corresponding motor winding by which a balanced voltage is applied to the motor so as to produce a single revolving'magnetic field, said reactor having a D. C. winding associated there- .with, and means continuously controlled by said resultant voltage so as automatically to adjust the current in said D. C. winding and thereby the balanced A. C. voltage applied to said motor substantially in proportion to variations in said resultant voltage to a value which maintains the motor speed substantially constant and proportional to said selected reference voltage.

-15. In a system for automatically maintain- E24 --.:ing:the speedrof'raniArnC:zzinductiommotor:atany effective impedance athereof equally :L-in :z-each 'gipjfes'eiected speed, .';the gcomloinati'on' which inphase, and circuit.meanscoupling said'icircuit :cludes,:an A; C. induction:rnotor:proportionedin element to the adjusting meansforsaidssource rrespect toits 'load'sdasto operate at substantialof direct current"forautomatically ad'justingzthe "1y one-third 'of hits norm-a1;v single-speed motor 5 magnitude of saiddirectcurrent in; response to rated torque,iandi proportionedwin respect to its changesi insaid difierence voltage by whichit-he rotor: resistance so asttoioperateatsitsxminimum current in each 111C. winding is automatically running speedatxacurrent which will not exadjusted as a function of said dilierencevoltage ceed its normahsingle-speed. motorrrated curto a value which maintai-ns'the eifective impedrent and at raimaxirnumi'speed' of approximately to ance of all said'reactor means, and thus-iof'ithe ninety percent of its synchronousspeed, a source balanced A. C. voltageiapplied-td said motomat of:alternatingmurrent pcwer for providing a 'bala magnitude whichrimaintains thessmotorzspeed aanced"voltagericonnectibietooperate said motor, at any desired ratenpreselectable by adjustment said motorf havingA as many stator windings as of said reference voltage.

therexarephaseswoftzsaidrsource;and means for 17. In a system forpreselecting anyvv speedcof "controlling the':motor; speed by varying the balan-A. C. induction motor over alarge-range in ancedzvoitage 'a gipliecrv to said" motor comprising speed and automatically maintainingthezselectsaturable"core'reactor: means-connected between ed speed, a source or alternating-currentrupower eacn phase of saidcpower' source'and a correfor providing a balanced voltage connectibleito sponding stator windingsofsaid motor by which operate said .motorusaid motor having: aszmany 'said balanced roltagepis applied to'the motor s0 stator windings :as there are phasestof. rsaid as'i to vproduce-a single-"revolvingimagneticfield, source, and means for *controllingrthe r motor -a D.1C.":windingin4 each'reactor means, a source speed by varyingthebalaneed voltage tapplied'tto -'.of contro1 voitageigenerated as a function of the said motor comprising *saturable :core reactor :speed" ofsaidcmotoryasource of adjustable refmeans: includingca-coil :connected betweenrseach erence'tvoltage, avcircuitelement in which said phase of said power. source-land a corresponding -control and reference voltages are-combined to stator winding of said:motor-by-awhich said'xbaltorm a'effe'ctively 'azdifferent voltage,xan adjustanced voltage its-applied" to the:;motor,:'so'.,.as to 'ablexsourceipf direct current connected to the produce .a single revolvingimagnetic"field;.a"D:.C. -D."C. -Winrling of 133611 saturable core reactor winding-associated =with-each reactor meansy a *means'iso "as to vary the effective impedance source of controlvoltageagenerated:asi-a; function tIISIBOfSQUZ HYiiH each phase, and circuit means of the speed of said motor, a source =of -adjustable -coupling: said. circuit element to the adjusting reference voltage; a circuit-elementdrrwhichi-said ---meansior"saidsourceiof' direct current for autocontrol and reference voltages-are combinedato "m t a y i s thexm en t devof aid dir c 3 form effectively-atdifferenoe voltage a' sourcezof 1current'inz'responseito changes insaid difference direct :current connected tozsaidD. C. windings, ivoitageby'whichthe'current in each-D. C. Wi means for adjusting said direct-currentisoz'asito ing is automaticallyadjustedras a function ofsaid vary th effective impedance of said =TeactQr difierenceivoltageito a value: whiclrmaintains the means equally in: each phase; andreircuit: means *fi fit e p dimccmf all said 'react an in coupling said-circuit; element utoi-thet adju'sting vandthusioi the balanced A. C vol a e pp 130 'means for said direct current' for: automatically #S d r, a magnitude whicnmaintains' th adjusting the'magnitude' of said 'directrzcurrent motor: speed at anydesired rate Preselectable by in response tozchangesiin' saidadifierence'voltage zrad'justment" tit-said-reference voltage. by which the :current. in each 1). TC. windingi is 16- In a y t m r au om l Imaintaining, automatically adjusted asafunctioni'of saidtdif- :thespeed'of anA. C; induction-motor at any prefe e voltage to value. whichgmamtamstthe *selectedspeed, :thewcombination'which includes, effe tive impedance 0ftsaid reactgr"meana and ra-n'A. iCiinductiorr motor proportioned in respect t of the balanced C voltage a lied xo ajid toiits load-so as to operate-at less than itsrnormal otor, at a lmagnjtude which m p d" l'fltfid q and 1 1 3 motor speed'atanyidesiredn-ate preselectableby "ticneddnfiespec't tO'itSlOtOI resistance 50 as 130' {adjustment of aidreference'vo'ltage p fis" e 8; 011T- 18. In combination, a 'multi-rphasewalternat- -rent-whi :h willxnot exceed-its normal .ingecurrent motor'rhavinga high=resistancerotor 1 1 fid atf'a maximum substantiahy: as 'describedpa'wmulti=phase source p 0f"EDDYOximatclyflinety'percent of its of alternating-current 'power for'providingz'sa r chr n s sp d Sourceof alternating-current balanced'voltage'c'onnectibleto operate said mo- 'p W 1-'f providing??? balanced-voltage co tonsirhilarsaturabie core reactor; means includiblerto'operatesaid motor, saidmotor having as mg a 1 connectedin each phaselbgtweenisaifld v man-y stator windings asthere' arephases of said power source i moltgrby w i balanced r and means for 'ccntl'ol ithe motor voltagetis applied to"the'motor'so: as to :produce p y y balancedfvoitage pp to a single revolving-magnetic-field, aaDnC's windsaid moto omp Saw-Table core reactor ing' associated with each saidireactor:means-ca means connected betw each phase of 5am source of control voltage generated'in: proportion PGWBI Source and a'cclllesponding Stator Winding 'to the speed of said motor; a source ofiadjustable of' said m B r y ch Said balanced V ltage is reference voltage; aicircuit element in'which said pp as t0 produce a Single control and. reference *voltages are vco'mbined'to "w g "magnetic fie W d form effectivelyadifierence voltageia sourcei'of reactor means; a source" of control Vo a direct current connectible-to'thel D.-rC: windings erated -asta function'of the specdfof' aid m of said saturablecore'reactor'meanssoiaszto cone; sourceof adjustable reference voltage, a circuit t the effectivewjmpedance said 1-eact0r element which said control and reference voltmeans, circuit meang-coup1ing;-sajd -circuit .ages are combined to formeffectively a difment to said source of direct current andopera- .re-rencewoltage, an adjustable :source of direct :tive automatically to adjust-the magnitude :of currentconnected: to: the .D. .C. winding oteach said, direct current in-- response tochangessin said ssaturable core"reaetorzmeans"sonas to jvaryzthe indifference voltage .bywhichtthe currentw-inwsaid D. C. windings is automatically adjusted as afunction of said diiference voltage to a value which maintains the effective impedance of said reactor means, and thus the balanced A. C. voltage applied to said motor, at a magnitude which maintains the motor speed at a rate determined bythe adjusted value of said reference voltage.

19. In a system'for controlling the speed and direction of rotation of a multi-phase A. C. induction motor having a winding for each phase, a first set of saturable core reactors including an A. C. winding for each phase and D. C. coil means associated therewith, one each of said A. Cnwindings being connected between each phase of the line and one each of said motor windings, respectively, a second set of 'saturable core reactors similar to those of said first set including an A. C. winding for each phase and D. C. coil means associated therewith, the A. C. windings of each of said second set of reactors being also connected between each phase of the line and one each of said motor windings, respectively, said second set of windings being connected tosaid phases in reverse order with of said first set, means autorespect to those matically generating a D. C. control voltage which is a function of the speed of said motor, speed and direction-of-rotation selecting means including a first sourceof adjustable D. C. reference voltage connected in series with the eifec- 1 tive output of said control voltage generator so as to produce a resultant voltage comprising the algebraic sum of said control voltage and said reference voltage, circuit means including amplifier means having a control element and a unidirectional current output connected to energize the D. C. coil means of the first set of said reactors, coupling means effectively coupling said resultant voltage to said control element of said amplifier means so that the unidirectional current in said D. C. coil means is automatically adjusted substantially in proportion to the magnitude of said resultant voltage to a value which maintains the effective impedance of said first set of reactors, and thus the balanced A. C. voltage applied to said motor, maintains the motor speed tional to the preselected value of said reference voltage, and second circuit means similar tosaid first-named circuit means similarly connected to the D. C. coil means of said second set of reactors including a second source of adjustable ranged to simultaneously adjust said first reference voltage to said preselected value whilst maintaining the second reference voltage at a predetermined minimum value, and vice versa With respect to operation of said motor in the reverse direction. l

20. A circuit for controlling the direction and speed of rotation of. a multi-phaseinduction motor comprising a multi-phase'power circuit for the motor; a first and .a second set of windreference voltage, said selecting means being arings, one winding of each set being connected in' series with the motor in eachline wire of the power circuit, with the windings of the different sets being connected in an altered manner so as to control the phase sequence of the voltage applied to the motor to govern the direction of rotation thereof, eachof said windings normally having a highimpedance preventing the appli-- cationof any effective voltage on the motor; a means for each set of windings operable upon energization to reduce the impedance of the windings or the e with which it-ls associated at a magnitude whichsubstantially propor in proportion to the degree of energization of the means, and control circuit means for selectively and variably energizing said first mentioned means, said control circuit means including adjustable means for generating signals in said control circuit corresponding to the desired direction and speed of rotation of the motor and operation responsive means operable to generate signals of a character and magnitude corresponding to the direction and speed of rotation of the motor and electrically interconnected in said control circuit means to maintain operation of the motor at a speed substantially as called for by the adjustable means.

21. A circuit for controlling the speed and direction of rotation of a multiphase induction motor, comprising a multiphase power source for the motor, said motor having as many windings as there are phases of said source, a plurality of reactors, each reactor including A. C. windings connected between each phase of said source and a corresponding winding of said motor, said A. C. windings being arranged in two sets, the windings of the difierent sets being connected in an altered manner so as to control the phase sequence of the voltage applied to the motor to govern the direction of rotation thereof, said A. C. windings normally having a high impedance preventing the application of any effective voltage on the motor, D. C. coil means associated with each set of A. C. windings operable upon energization to reduce the impedance of the A. C. windings of the set with which said D. C. coil means is associated in proportion to the degree of energization of said D. C. coil means, control circuit means for selectively and variably energizing said D. C. coil means, said control circuit means including adjustable means for generating singals in said control circuit corresponding to the desired direction and speed of rotation of the motor, and operation-responsive means operable to generate signals of a magnitude cor responding to the speed of rotation of the motor and electrically interconnected in said control circuit means to maintain operation of the motor at a speed substantially as called for by the adjustable means.

22. A circuit for controlling the speed of rotation of an A. C. induction motor, comprising an A. C. power source for the motor, said motor having as many windings as there are phases of said source, a reactor including an A. C. winding connected between each phase of said source and a corresponding winding of said motor, each A. C. winding normally having a high impedance preventing the application of any effective voltage on said motor, D. C. coil means associated with said A. C. windings operable upon energization to reduce the impedance of each A. C. winding in proportion to the degree of energization of the D. C. coil means, control circuit means for variably energizing said D. C. coil means, said control circuit means including adjustable means for generating signals in said control circuit corresponding to the desired speed of rotation of the motor, and operation-responsive means operable to generate signals of a magnitude corresponding to the speed of rotation of the motor and electrically interconnected in said control circuit means to maintain operation of the motor at a 27 varying load, a source of A. C. power for said motor, said motor having as many windings as there are phases of said source, means for generating a speed-responsive voltage which is a function of the speed of said motor, speed-selecting means including a source of adjustable reference voltage connected in said system effectively in series with said speed-responsive voltage so as to produce a resultant voltage comprising the algebraic sum of said speed-responsive voltage and said reference voltage, saturable core reactor means having an A. C. winding connected betweeneach phase of said source and a corresponding motor winding, by which a balanced voltage is applied to the motor so as to produce a single revolving magnetic field, each A. C. winding normally having a high impedance preventing the application of any effective voltage on said motor, said reactor mean having a D. C. winding asso ciated therewith, and means continuously controlled by said resultant voltage so as automatically to adjust the current in said D. C. winding and thereby the balanced A. C. voltage applied to said motor as a function of said resultant volt-- age to a value which maintains the preselected motor speed substantially constant.

24. In a system for preselecting any speed of an A. induction motor over a large range in speed and automatically maintaining the selected speed, a source of alternating-current power for providing a balanced voltage to operate said motor, said motor having as many windings as there are phases of said source, means for con trolling the motor speed by varying the balanced voltage applied to said motor comprising saturable core reactor means including an C. winding connected between each phase of power source and a corresponding winding of said motor by which said balanced voltage i applied to the motor so as to produce a single revolving mag-- netic field, each A. C. winding normally having a high impedance preventing the application of any effective voltage on said motor, a D. C. winding associated with each reactor means, a source of control voltage generated as a function of the speed of said motor, a sourceof adjustable ref element in which saiderence voltage, a circuit control and reference voltages arecombined to form effectively a difference voltage, a source of direct current connected to each D. C. winding, means for adjusting said direct current so as to vary the effective impedance of said reactor means equally in each phase, and circuit means coupling said circuit element to the adjusting means for said direct current for automatically adjusting the magnitude of said direct current in response to changes in said difference voltage by which the current in said D. C. winding is automatically adjusted as a function of said difference voltage to a value which maintains the eifective impedance of said reactor means, and thus of the balanced A. C. voltage applied to said motor. at a magnitude which maintains the motor speed at any desired rate preselectable by adjustment of said reference voltage.

25. In combination, a multiphase alternatingcurrent motor having a high-resistance rotor substantially as described, a source of multiphase alternating-current power for providing a bal anced voltage to operate said motor, saturable core reactor means including an A. C. winding connected in each phase between said power source and said motor by which a balanced voltage is applied to the motor soas to produce a single revolving magnetic field, a D. C. winding associated with each said reactor means, a source of'control voltage generated in proportion to the speed oi! said motor, a source of adjustable reference voltage, a circuit element in which said control and reference voltages are combined to form. elfectively a difference voltage, a source of direct current connected to the D. C. windings of saidreac tor means, and means for adjusting said direct current so as to vary the effective impedance of said reactor means, circuit means coupling said circuit element to said source of direct current and for automatically adjusting the magnitude age applied to said motor, at a magnitude which maintains the motor speed at a rate determined by the adjusted value of said reference voltage.

26. A circuit for controlling the rotation of an A. C. induction motor, comprising an A. C. power source for the motor, said motor having as many windings as there are phases of said source, a reactor including an A. C. winding connected between each phase of said source and a corresponding winding of said motor, each A. C; winding normally having a high impedance preventing the application of any effective voltage on said motor, D. C. coil means associated with said A. C. windings operable upon energization to reduce the impedance of each A. C. winding in proportion to the degree of energization of the D. C. coil means, control circuit means for varl-- trol circuit means. including adjustable means.

for generating signals in said control circuit cor responding to the desired speed of rotation of the motor, operation-responsive means including direct-current generator means driven by said motor and operable togenerate signals of a magnitude and polarity corresponding respectively to the speed and direction of rotation of the motor and electrically interconnected in said control circuit means to maintain operation of the motor at a speed substantially as called for by the adjustable means.

27. A circuit for controlling the speed and direction of rotation of a multiphase induction motor having a high resistance secondary, comprising a multiphase power source for the motor, said motor having as many windings as there are phases of said source, a plurality of reactors, each reactor including A. C. windings connected between each phase of said source and a corresponding winding of said motor, said A. C. Windings being arranged in two sets, the windings of the different sets being connected in an altered manner so as to control the phase sequence of the voltage applied to the motor to govern the direction of rotation thereof, said A. C. windings normally having a high impedance preventing the application of any effective voltage on the motor, D. C. coil means associated with each set of A. C. windings, each coil means being operable upon energization to reduce the impedance of the. A. C. windings of the set with which it is associated in proportion to the degree of its energization, control circuit means for selectively and variably energizing said D. C. coil means, said control circuit means including adjustable means for gen crating signals in said control circuit means corresponding to the desireddirectionandspeed of rotation of the motor, and operation-responsive said reactor means, and thus the balanced A. C. volt 29 means operable to generate signals of a magnitude corresponding to the speed of rotation of the motor and electrically interconnected in said control circuit means to maintain operation of the motor at a speed substantially as called for by the setting of the adjustable means.

References Cited in the file of this patent UNITED STATES PATENTS Number 30 Name Date Stoller July 2, 1929 Wilson Dec. 30, 1930 Higbee Nov. 10, 1931 Thompson Feb. 9, 1932 Wesche Jan. 17, 1933 Seeger et al Feb. 20, 1934 Brown Apr. 17, 1934 Logan Apr. 27, 1937 Perry Dec. 21, 1937 Gulliksen Jan. 7, 1941 Logan Oct. 27, 1942 Wickerham Apr. 27, 1948 

